1. Field of the Invention
The present invention relates to the control of the exothermic heat of reaction generated in an exothermal reactor system and, more especially, to the removal of the exothermic heat of reaction generated in a large scale exothermal reactor system such as a digester for the production of wet-process phosphoric acid.
2. Description of the Prior Art
It is well known to the art of wet-process phosphoric acid production, for example, that relatively large quantitites of the exothermic heat of reaction must be exactingly removed in order to maintain the desired temperature in the reactor/digester system. And this is typically sought to be accomplished either by circulating a fraction of the reaction slurry through a vacuum cooler (compare U.S. Pat. Nos. 3,257,168, 3,472,619 and 3,552,918) or by maintaining the reactor itself under vacuum (compare U.S. Pat. Nos. 3,416,889, 3,522,003, 3,522,004, 3,666,413 and 3,917,457).
Moreover, due to certain technical problems associated with the above and the energy efficiency thereof, it too is known to effect such cooling by blowing air over the surface of the reaction mixture in the reactor.
For a more detailed description of the temperature control problem and the various proposed solutions thereof, see A. V. Slack, Phosphoric Acid, 1, Part I, pp. 227-231, Marcel Dekker, Inc., New York (1968).
Further, the air cooling alternative avoids such disadvantages of the use of a vacuum cooler as scaling, the need for periodic washing, the creation of a temperature gradient detrimental to good crystallization; air cooling similarly avoids such disadvantages of maintaining the reactor under vacuum as the requirement for airtightness and concomitant difficulties in apparatus construction, as well as the increased size of the installations by reason of the amount of gaseous species which do not condense.
In contrast, the cooling air charged with water vapor, carbon dioxide, fluorine, etc. (compare A. V. Slack, Phosphoric Acid, 1, Part II, pp. 744-745, Marcel Dekker, Inc. New York (1968)), must be scrubbed before being discharged back into the atmosphere; such treatment mandates additional capital investment due to the high flow rates involved and also because of the need for extensive purification which may necessitate the use of several installations, giving rise to the expenditure of additional energy, and by reason of the discharge of gases which are still hot, heat loss also characterizes the air cooling system.